The present invention generally relates to optical semiconductor devices. Especially, it related to optical semiconductor devices such as a semiconductor optical modulator, a semiconductor light-emitting device, a semiconductor optical amplifier, an optical source of amplitude spontaneous emission (ASE), an optical gate array; a tunable laser apparatus, a multi-wavelength laser apparatus, and also an optical transmission system that uses such an optical semiconductor device. Further, the present invention relates to the fabrication process of the optical semiconductor devices.
Japanese Laid-Open Patent Publication 10-22805 describes a semiconductor optical modulator that uses an InGaAsP multiple quantum well structure. In this reference, an InP/InGaAsP multiple quantum well structure is used as an optical modulator, and optical modulation is achieved by causing quantum confinement Stark effect in the multiple quantum well structure. In quantum confinement Stark effect, there occurs a decrease of energy bandgap between the electrons and holes forming excitons upon application of a voltage to the quantum well layer.
Thus, when a voltage is applied to a quantum well structure having zero optical absorption in the state that no voltage is applied thereto, there occurs an increase of optical absorption as a result of decreased bandgap in the quantum well layer, and there is caused optical absorption associated with such a decrease of bandgap. Thereby, there is formed an EA (electro absorption)-type optical modulator.
However, conventional EA-type semiconductor optical modulator that uses the InP/InGaAsP multiple quantum well structure has a drawback, due to the relatively small conduction band discontinuity between the InP barrier layer and the InGaAsP quantum well layer, in that the excitons are tend to be destroyed due to the leakage of electrons from the quantum well layer upon application of the voltage to the quantum well layer, wherein this problem becomes particularly serious when operating the EA modulator at high temperatures. In such a case, the magnitude of decrease of the bandgap, caused by the quantum-confinement Stark effect is reduced and hence the magnitude of change of the optical absorption. As a result, there arises a problem of severe degradation of S/N ratio of optical modulation.
Meanwhile, in the art of optical telecommunication, optical amplifiers are used extensively for compensating for the transmission loss occurring in optical fibers or for compensating for coupling loss of various optical components.
Conventionally, optical amplifiers of progressive wave type are used extensively. In an optical amplifier of progressive wave type, an optical beam incident to an end surface of the optical amplifier is amplified as it is propagated through an optical waveguide region in which a gain region, characterized by a gain with respect to optical radiation having a wavelength of the incident optical beam, is provided. Thereby, an amplified optical beam is obtained at an opposite end surface. In the gain region, holes injected from a p-side electrode and electrons injected from an n-side electrode form together a population inversion, and the incident optical beam induces stimulated emission as it is propagated through the gain region.
In the case of using a quartz optical fiber, the wavelength band of 1.2-1.6 μm is thought as the optimum wavelength for long distance optical telecommunication, in view of the minimum transmission loss at this wavelength. Japanese Laid-Open Patent Publication 9-105963 or 11-186654 describes a conventional propagating wave type semiconductor optical amplifier constructed on an InP substrate and having an InP cladding layer. According to these references, optical amplification at the wavelength of 1.3 μm or 1.5 μm is described by using a InGaAsP quantum well layer.
Thus, the conventional semiconductor optical amplifiers operable in the wavelength band of 1.2-1.6 μm have been constructed by growing a material layer of the InGaAsP system on the InP substrate. In such a system, however, there arises a problem similar to the case of the EA-type optical modulator in that, because of the relatively small conduction band discontinuity between the InGaAsP layer acting as the gain region and the InP layer acting as a carrier-blocking layer of 150-200 meV, and further in view of the Auger non-optical recombination effect, and the like, the electrons easily cause leakage from the gain region. The problem of electron leakage becomes particularly serious at high temperatures.
In the case of the InGaAsP laser diode operable at the wavelength of 1.3 μm, it should be noted that the characteristic temperature has a value of about 80K, while this value of characteristic temperature is about one-half the characteristic temperature of an AlGaAs laser diode operable at the wavelength band of 0.85 μm.
The same tendency applies also to the case of the semiconductor optical amplifier, and thus, there is a tendency that optical amplification causes saturation at high temperatures as a result of the leakage of carriers from the gain region. As a result of such a carrier leakage, it has been difficult to achieve a large optical amplification factor in the conventional semiconductor optical amplifiers particularly at high temperatures.
Thus, this problem has been a bottleneck when constructing a low-cost optical LAN system that requires semiconductor optical amplifiers without using an electronic cooling system.